1. Field of the Invention
This invention relates to a high energy accelerator and in particular to an accelerator, which accelerates charged particles by using electro-magnetic waves of high frequencies.
2. Description of the Related Art
As charged particle accelerators, linear accelerators, loop-shaped storage rings, etc. are known. In these accelerators a high frequency electric field (electro-magnetic wave) is used for accelerating charged particles to an extremely high energy.
When microwave energy is made to pass through a simple cylindrical structure, the phase velocity of the microwave exceeds the velocity of light. It is a matter of course that the energy propagation velocity (group velocity) cannot exceed the velocity of light. Therefore, in order to accelerate charged particles in the neighborhood of the velocity of light, the structure of an acceleration tube is so designed that the phase velocity of the microwave propagating therein is caused to be lower than the velocity of light, so that the phase velocity and the velocity of the charged particles are equal. More specifically, a structure varying periodically along the axis of the acceleration tube is adopted so that there exists a certain dispersion relation based on the periodicity between the energy and the wave number vector.
Hereinbelow, unless otherwise specified, as an example, a linear accelerator for accelerating electrons will be explained. The prior art structure of the linear accelerator is described e.g. in IEEE Trans. Nuclear Science, NS-28, No. 3, P. 2873, p3440, June 1981.
Now, as the first index representing characteristics of a linear accelerator Em/Ep is used, in which Ep represents the possible maximum field strength, which can be considered to be a constant, the upper limit of which is determined by discharge inside of the acceleration tube. Consequently, in order to increase the spatially averaged electric field Em for accelerating charged particles, a structure, in which Em/Ep is great, i.e. as close as possible to 1, is preferable. When Em is also averaged with respect to time, the maximum value of the ratio .mu. becomes 1/.sqroot.2 . When the accelerating electric field Em is restricted to a small value, a longer acceleration tube is necessary for accelerating charged particles to a certain energy and according to circumstances the acceleration tube would be so long that it is difficult to realize it.
As the second index representing characteristics of the linear accelerator, v.sub.g /C is used. When charged particles are accelerated, energy of the microwave supplied to the acceleration tube is consumed by particles to be accelerated. Consequently, in order to continue the acceleration of the particles, it is necessary to supply smoothly energy of the microwave. For this purpose it is required that the energy propagation velocity (group velocity of the microwave) v.sub.g is great. Since the velocity cannot exceed the velocity of light C, a dimensionless parameter v.sub.g /C is used for analyzing characteristics of a linear accelerator, adopting the velocity of light C as a reference value. Then it is desirable that v.sub.g /C is as close as possible to 1. In order to be able to accelerate particles in the neighborhood of the velocity of light a simple periodic construction is insufficient and a combination of a plurality of periodic structures is efficacious. It is, for example, a composite periodic structure including disks on the inner surface of the cylinder wall and washers separated from the inner surface of the cylinder wall.
As the third index representing characteristics of the linear accelerator the effective shunt impedance R is used. This is a measure for the acceleration efficiency, indicating with what efficiency injected energy of the microwave contributes to the acceleration of the particles, and a greater R can be interpreted to be a higher acceleration efficiency.
The effective shunt impedance per unit length can be defined as follows; ##EQU1## where
L: length of a period of the acceleration tube,
E.sub.0 (Z): strength of the electric field on the axis of the beam,
P: energy loss due to wall current for the length of a period,
.lambda.: guide wavelength of the microwave, and
v: velocity of particles.
That is, in order to increase the effective shunt impedance, it is efficacious to increase the axial component of the electric field vector on the axis and to reduce the energy loss P due to wall current.
If simply disks protruding from the inner surface of the acceleration tube towards the axis are disposed, the shunt impedance is small and thus the acceleration efficiency is low.
Accordingly it has been proposed to use thin washers separated from the inner surface of the acceleration tube in order to reduce the wall current and to dispose projections in the axial direction in the central aperture portion of the washers in order to strengthen the electric field on the axis. However, if importance is attached only to strengthening the electric field on the axis, the electric field is reduced as a whole in the acceleration tube. Consequently the length of the acceleration tube for accelerating particles to a predetermined energy is increased to such an extent that it is unattainable.